The present invention relates generally to asynchronous transfer mode (ATM) networks and other types of packet-based communication systems, and more particularly to techniques for transmitting voice and telephony information, as well as other types of information, over such systems.
An ATM communication system provides high-speed, low-delay switching of voice, data, video and other types of user information traffic. In an ATM system, the user information traffic is separated into fixed-length 53-byte cells. Each ATM cell typically includes a 5-byte header and a 48-byte payload. The header incorporates a virtual channel identifier (VCI) and a virtual path identifier (VPI) associated with the corresponding cell. The VCI and VPI together specify a virtual connection (VC) which is established when a user requests a network connection in the ATM system. Additional details regarding these and other aspects of ATM systems can be found in the ATM Forum, xe2x80x9cATM User-Network Interface Specification,xe2x80x9d Version 3.1, September, 1994, and in Martin de Prycker, xe2x80x9cAsynchronous Transfer Mode: Solution for Broadband ISDN,xe2x80x9d Ellis Horwood, New York, 1993.
Communication between an ATM layer and upper layer devices, such as ATM Adaptation Layer (AAL) devices, is specified in the above-cited User-Network Interface (UNI) reference. In a conventional system for transmitting voice and telephony over ATM (VTOA), voice samples are transmitted in ATM cell payloads, while a separate well-known ATM channel, i.e., a channel designated by VCI =5 and VPI =0, is used as a default channel to transmit the corresponding telephony signaling information. This telephony signaling information may include, for example, on-hook/off-hook and ring/no-ring indicators. The ATM Forum, xe2x80x9cVoice and Telephony Over ATM to the Desktop Specification,xe2x80x9d AF-VTOA 0083.000, May 1997, specifies the use of the default channel for transmitting telephony signaling information in VTOA applications, and the above-cited UNI reference specifies that this default channel is the VCI =5, VPI =0 channel.
The above-noted default signaling channel generally serves as a common signaling channel for multiple users accessing the same connection end points. As a result, the signaling channel can easily become overloaded, leading to a bottleneck in transmission of VTOA. In addition, the telephony signaling information in the conventional signaling channel generally must be examined by intermediate switching elements in the connection path, even though the telephony signaling information is typically only required by the connection end points. This unnecessary processing can lead to further performance degradations. Existing techniques for transmitting VTOA thus fail to provide simple and efficient transmission of telephony signaling information over the ATM layer, and can significantly undermine the performance of voice and telephony applications in ATM or other packet-based systems.
The invention uses packet headers in an ATM or other packet-based communication system to transmit signaling information. In an illustrative embodiment of the invention, telephony signaling information, such as an on-hook/off-hook indicator, a ring/no-ring indicator, or any other type of information used in establishing, maintaining, terminating or otherwise configuring a telephony-based communication, is communicated over an ATM link using ATM cell headers. For example, a single bit of telephony signaling information may be incorporated into a low order bit, i.e., a least significant bit (LSB) of a packet type indicator (PTI) field in a header of an ATM user data cell. Such an arrangement is possible because voice samples associated with the telephony-based communication are generally single-cell messages, such that the LSB portion of the PTI field in the header, which could be used to indicate an End of Message (EOM) cell in a multi-cell message, can instead be used to transmit the signaling information. Other embodiments of the invention may utilize, for example, a cell loss priority (CLP) bit in the cell header to transmit signaling information in applications in which the CLP bit is end-to-end unmodified for a given connection. The CLP bit may be used in conjunction with the LSB of the PTI field to provide a multi-bit signaling channel, or may be used to provide a separate signaling channel independently of the PTI field.
In embodiments of the invention in which the transmitted signaling information is telephony signaling information, out-bound telephony signaling information received from a telephony device such as a telephone or a computer modem is incorporated into an ATM cell header, transmitted with the cell over an ATM link, removed from the header in an ATM processing device, and delivered to a public switched telephone network (PSTN). In-bound telephony signaling information received from the PSTN is incorporated into an ATM cell header, transmitted with the cell over an ATM link, removed from the header in an ATM processing device, and delivered to the telephony device. The ATM processing devices may be implemented, for example, as telephony-to-ATM converters or suitable interworking functions, and may be incorporated into switches, PBXs, or other elements associated with a packet-based communication system.
Although particularly well suited for use in transmission of telephony signaling information over ATM systems, the invention may also be implemented in other types of packet-based communication systems, using other types of signaling information, and with a wide variety of different system configurations. Advantageously, the invention can transmit signaling information in a simple and efficient manner, without interfering with the conventional functions of the header. These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.